In an internal combustion engine, when fuel leaks from a fuel injector, when fuel is blown back from an combustion chamber toward the intake port, or when fuel flows out from a positive crankcase ventilation passage, the fuel containing HC may remain around an intake port with the engine off. A multi-cylinder internal combustion engine, especially, has a remaining fuel in an intake manifold. After the engine is stayed for a certain period in such a state, the remaining fuel evaporates into a floating HC. When the engine is re-started with the floating HC, the floating HC around the intake port is introduced into the combustion chamber and then exhausted as an unburned gas.
To avoid such a problem, some apparatuses have been introduced. JP-11-82192A shows a conventional apparatus in which an absorbent is provided between a throttle valve and an engine in order to absorb the fuel leaking from injection valves. In JP-2001-227421A, a HC absorbent is provided in an intake pipe for absorbing the HC remaining in the intake pipe. JP-2001-234781A shows an apparatus in which the HC remaining in an intake pipe is absorbed in a HC absorbent temporally and then the absorbed HC is purged after activation of a catalyst or after a certain period passed from starting of an engine.
Each of the conventional apparatuses described above has the HC absorbent in the intake pipe of the engine. However, the absorbed HC having a high boiling point is not purged easily. Thus, an absorbing characteristic of the HC absorbent is deteriorated and a sufficient reduction of HC is not achieved.
In JP-2001-227421A, the HC absorbent is disposed upstream of a throttle valve, for example, in an air cleaner, and a canister for absorbing a fuel evaporated in a fuel tank is utilized as the HC absorbent. Since a floating distance of the HC having a high boiling point is shorter than that of the HC having a low boiling point, the HC having a high boiling point does not reach the HC absorbent while the engine is on. On the other hand, when the HC absorbent is disposed upstream of the throttle valve or when the canister is used as a HC absorbent, the HC having a low boiling point around the intake port is not reduced effectively while the engine is stopped. Thus, when the engine is cranked, the floating HC around the intake port is introduced into the combustion chamber and is exhausted as an unburned gas.